The present invention generally relates to reticle technology, photolithography, and ASIC manufacturing, and more specifically relates to a Pseudo Low Volume Reticle (PLVR) design for ASIC manufacturing.
A number of problems have been identified in the fields of reticle technology, photolithography and ASIC manufacturing, including: the high cost of a high volume reticle (HVR) mask set due to the total number of masks needed for a device; the inability to inspect the scribe region of an HVR set by die-to-die inspection techniques; the high cost of defect inspection on Low Volume Reticles (LVRs) due to die-to-database restrictions; the need for LVRs to be compatible with a die-to-die defect inspection tool; the long overhead time for data preparation time in die-to-database inspection; the long overhead time associated with disposition of false defects from die-to-database inspections; the increase potential of mask shop re-write due to non yielding single die LVR reticles; the need for fast prototype turnaround time (TAT) associated with the first mask of an LVR reticle set; and the inability to perform in-line inspections of LVR reticles on a die-to-die inspection tool.
One existing approach includes using HVR reticles which are die-to-die compatible. In conventional HVR manufacturing, there exists one reticle for each layer of a device. Data for each layer is arrayed on each reticle to maximize the instances of die exposed per shot. While multiple instances of the die on the reticle allow for die-to-die inspection, the X and Y scribe are excluded from inspection due to a lack of reference structure. Additionally, this method is lacking for small scale ASIC production since mask sets with many layers are relatively expensive for low volume customer orders.
Another existing approach includes using LVR reticles which require die-to-database inspection, but which minimize the total number of reticles in a set. In a conventional low volume reticle (LVR) approach, each reticle consists of multiple layers of a device on a single reticle in order to minimize the total number of reticles and hence reticle set cost. Only one layer region is exposed at a time such that a single reticle is used for several masking steps in the device manufacturing process. For example, FIG. 1 illustrates a reticle 10 which is consistent with a conventional LVR approach, wherein four different layers or regions 12 (i.e., region A, region B, region C, and region D) are included on one reticle. As shown in FIG. 2, rather than expose the whole reticle field, the stepper file 14 blades off all but one region (region A) of the reticle 10 and one layer 16 is printed at a time (see FIG. 3). The second region (region B) of the reticle (see FIG. 4) is then used to print the next layer 18 of the device (see FIG. 5).
Hence, a conventional LVR approach reduces reticle cost by minimizing the total number of reticles in a set. The cost of a reticle set is reduced from standard HVR production proportional to the decrease in total number of reticles to make a design. However, while a conventional LVR approach takes into account the number of reticles in a set, a conventional LVR approach doe not consider the cost implications associated with mandatory die-to-database reticle inspection, reticle yield, and wafer yield.
LVR reticles are not die-to-die inspection compatible, a method which compared to die-to-database inspection, is less expensive, faster, and less prone to false defects. Specifically, since there are several layers rather than multiple instances of the same layer, there is no reference die for a die-to-die inspection. FIG. 6 illustrates why an LVR reticle cannot be inspected by die-to-die inspection methods. Since there is only one instance of each layer (i.e., one instance of each of layers A, B, C, and D), a reference for defect inspections must be provided from an outside data source.
Moreover, since LVR reticles contain only one instance of each layer, reticle defects have a higher potential of causing mask and wafer yield fallout. Since a conventional LVR contains only one instance of each layer, a defect 20 on a single layer instance, such as shown in FIG. 7 (region A has the defect 20 in FIG. 7), can potentially make the reticle 10a unusable. Moreover, if the defect is added while in production use, all devices across the wafer could be rendered defective as illustrated in FIG. 8.
The fact that LVR reticles are not die-to-die inspectible and present a higher risk of mask and wafer yield fallout are both important considerations which affect both the mask manufacturing cost and cost of ownership of a reticle set.